Apparatus for applying abrasives to grinding and smoothing mechanism



H. K. HITCHCOCK. APPARATUS FOR APPLYING ABRASIVES T0 GRINDING AND SMOOTHING MECH ANISM.

Patented Apr. 13, 1920.

APELLCATIQN FILEILJULY 1.9, 1918. 1,336,521

4 SHEETS-SHEET I.

WITNESSES H. K. HITCHCOCK. APPARATUS FOR APPLYING AB.RA.S|.VES T0 GRINDING AND S MOOTHING MECHANISM.

APPLICATlON FILED. JULY 19, 1918.

INVENTOR Patented Apr. 13

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WITN ESSES 2 H. K. HITCHCOCK. APPARATUS FOR APPLYING ABRAVSIVES T0 GRINDING AND SMOOTHING MECHANISM.

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APPLICATION FILED. JULY l9, I918.

Patented Apr. 13, 1920.

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(IJIIIIIII 1K Imam-oi APPARATUS FOR APPLYING ABRA-SLVES T0 GRINDING AND SMOOTHING MECHANISM.

WITNE s sss I-IALBERT K. HITCHCOCK, OF PITTSBURGH, PENNSYLVANIA.

APPARATUS FOR APPLYING ABRASIVES TO GRINDING AND SMOOTI-IING MECHANISM.

Originalapplication filed December 31, 1913, Serial No. 809,695.

To all whom it may concern Be it known that I, HALBERT K. HITCH- cooK, a resident of Pittsburgh, in the county of Allegheny and State of Pennsylvania, have invented a new and useful Improvement in Apparatus for Applying Abrasives to Grindin and Smoothing Mechanism, of which the following is a specification.

This invention relates to apparatus for handling, grading and applying finely divided abrasive material .to grinding machines, and particularly for grading and applying sand to ing machines.

This application is a division of my application filed December 31, 1913, Serial No. 809,695.

The invention relates to that class of apparatus wherein the finely divided abrasive material, such as sand, is separated into grades of different sizes while in suspension in a liquid, as described in my Patent No. 934,441, September 21, 1909, and, while in suspension in the liquid, is'circulated repeatedly from the grading vessel to the grinding machine and back again, as described in my Patents No. 934,612 of September 21, 1909, and No. 1,056,415 of March 18, 1913.

One object of the present invention is to provide apparatus whereby a measured charge of abrasive material sufiicient for completing a single grinding operation, such as facing and finishing a side of glass, may be applied as a single batch or portion and repeatedly circulated in suspension in a liquid from the grinding table to the grader and back again, being thereby gradually reduced to finer and finer grades which are supplied to the table in turn as the grinding operation requires finer and finer abrasive, thereby increasing the efficiency and shortening the time of the grind ng operation, avoiding liability of defects in the surface being ground, reducing the quantity and consequently the cost of the sand used, and also conserving the fine sand for finishing operations. I

A further object of the invention 1s to generally simplify and 1mprove the ent re apparatus, so that the space required for 1138 installation is reduced, its efiiciency 1s 1ncreased, the time for the grinding and finishing operations is reduced, and the liability Specification of Letters Patent.

glass grinding and smooth- Patented Apr. 13, 1920.

Divided and this application filed July Serial No. 245,655.

of injuring the surface being smoothed is lessened.

In the grinding and smoothing of plate glass, and other surfaces where abrasive material is applied, it has been found highly desirable to have the abrasive supplied at any one time as nearly uniform in size as possible. The harmful effects of improperly graded material are apparent even during the rough or coarse grinding, in which case if finer material is mixed with the coarse, it interferes with the most effective grinding action because to the extent that it displaces coarse material it decreases the total grinding action, and is also wasteful of abrasive material because such finer particles are being reduced to a still finer condition without producing any corresponding grinding effect. But it is during the stages of finer grinding that the effects of non-uniform grading are most harmful, as any coarse material mixed with the finer material produces deep crushes or sand holes in the surface being ground, thereby necessitating further grinding to take out such rough ground spots and greatly delaying the final finishing, or if not done the finished product is of an inferior quality.

In operating prior apparatus of this type it has been the practice to add fresh abrasive material, such as coarse sand, to the circulat ing system from time to time during the grinding operation. This is liable to produce non-uniformity in the grades and contributes to some of the harm ful effects above described. I have so perfected this system of grading and applying abrasives to grinding machines that the time required has been so shortened and the amount of abrasive used has been so reduced that the operator now has not time to accurately gage the amount of abrasive required from time to time, from merely observing the operation as it proceeds, for there is always a certain lapse of time after starting to feed fresh abrasive into the system before its effect is felt in the grinding operation, and likewise this effect continues some time after the supply of fresh abrasive ceases, all of which makes it advantageous to start the new abrasive into the system before the grinding begins and shut it off before it is completed. I have found that in order to get the best results it is necessary to have definite times for starting the fresh abrasive and introduce definite quantities of fresh abrasive into the system, so that when the grinding actually begins the system will have enough fresh abrasive to insure that none of the finer grades which will be needed later are used in the rough grinding or facing, as each grinding operation produces only a sufficient amount of fine abrasive to insure a perfect surface in the finished product.

I have found'that great reduction in the time of grinding and greater perfection in 'the product can be effected by carefully measuring the coarse or unused sand or other abrasive material and introducing it in a single batch, or at least rapidly, during the early stages of the rough grinding,

and in any event completing the introduction of the coarse abrasive material prior to the completion of the rough grinding or facing and the beginning of the smoothing or finishing operation. Thisresults in reducing the time of grinding for two reasons, first, it insures that only the coarsest of the new sand is used for the facing operation and thereby greatly reduces the liabilrough'grinding operation it is' a mere wastev of valuable material that will be needed later, and freduces the efliciency of the sys- ".Y Itis therefore essential for the greatest efii'ciencyto" arrange the apparatus and so operate it that it will produce as uniform grades as possible and'to prevent as fas as possible the mixture of different grades or rial to the table.-

sizes and to preserve during the entire grinding operation as nearly uniform asposslble those. conditions which afl'ectv either the grindingoperation or the supply of mate- The present, invention is intended to secure the beneficial results above specified and to provide apparatus whereby all of the abrasive material, both the original supply and the used material, is graded or regraded under uniform conditions. T 0 this end each grading'rapparatus is arranged to take care ...-.-of but a single grinding table, and the grading is effected under a constant .or uniform head of liquid, thereby insuring that in the grading vessel the several grades will never vary materially from a' given level, and any given grade may be supplied with out change as long as desired.

To the same endthe abrasive material single batch or portion at the beginning of the operation, so that it becomes thoroughly graded before the finer grinding operations begin. The abrading material is kept constantly in suspension in the liquid and is circulated from the grading vessel to the grinding table and back again into the grading vessel and there regraded, following this cyclerepeatedly through the entire course of the grinding operation. On the table most of the corse sand is broken and reduced in size between the runners and the glass and by the grinding actions between the particles themselves, but a smallquantity of the coarse sand is liable to be thrown from the table without being broken or reduced in size, and especially while the runners are being. supplied to the glass and before the runners are fully down. Every time the sand passes over the table some of it is reduced to particles too small for further use,'and such material together with the scum and slime are separated out during the course of each cycle, the useful abrasive material alone being retained and regraded each time it returns to the grading vessel, from whence it is returned to the table as long as it is coarse enough to be useful in that stage of the grinding operation then r proceeding. As the material gradually becomes' finer and finer, it is automatically separated out in the grading vessel into the proper grades, and is-therein held back to be used during the successive stages of finer and finer grinding.

The invention may be embodied in various forms ofapparatus, one suitable arrangement being shown in the drawings, but it is understood that this is merely illustrative and does not limit the invention to the apparatus shown. In the drawings Figure 1 is a plan view of the apparatus; Fig. 2 is an elevation thereof, the sump being show-n in section; Fig. 3 is a sectional elevation of the grading vessel taken on the line 3 3, Fig. 1, and looking in the direction of the arrows; Fig. 4 is a similar section taken on the line 4- 1, Fig. 1 and'looking in the direction of the arrows; Fig. 5 is a detail view of a suitable indicating device; Fig. 6 is a detail plan L .view of the outlet from the grading vessel;

Fig. 7 is an elevation thereof viewed from the inside of the tank; and Fig.8 is a sectiolii'azl elevation on the line 8-8, Figs. an I y In thedrawings 1 indicates in conventiofil sign a typical form of glass grinding table provided with a pair of runners or rubbing members 2 of the usual form.

These parts, however, are merely typical of one form of grinding apparatus which may be used.

The table 1 is surrounded by a gutter 3 connecting with a cross gutter 4 which carries the liquid and used abrasive from the table over a screen 5 to the pit or sump 6 from which it is drawn through pipe 7 by means of a suitable pump, such as a centrifugal pump 8, driven by a suitable motor, such as the electric motor 9, and is forced through pipe 10 upwardly through the bottom of the grading vessel 11, thereby producing in said vessel an upward flow of the liquid relative to the abrading material and producing a separation of the material into grades of different sizes, depending upon the-ratio ofvthe weight of the particles to the resistance to dropping offered by the upward current of liquid, as is described in my Patent No. 934,441, above identified.

The grading vessel 11 is designed to grade the new material and also regrade all of the used material and to continuously maintain in said vessel at constant or uniform levels all of the different grades of material above a size too fine for further use. The material too fine for further use, including the dirt and scum, escapes constantly through overflow pipe 12 leading to the ditch or sewer 13, which also receives the overflow from the sump 6.

The fresh abrading material, such as coarse sand, may be added to the system at any point, either into the grading vessel 11 or upon the table 1, but preferably is supplied to the sump 6. As shown it'is delivered by the clam-shell bucket 14 into a suitable bin or hopper 15 having a valved outlet 16 arranged to deliver the sand into a measuring box or reservoir 17 large enough to hold suflicient sand for one complete grinding operation. The measuring box 17 has a sloping bottom and is provided with a valved outlet 18 arranged to deliver sand upon the screen 5 and into the sump 6. Preferably, a water supply pipe 19 having a number of small side openings is placed in the box 17 so that water may be supplied thereto to wash the sand quickly into the sump and get it into the circulation system before the finishing operation begins.

The grading vessel 11 may be of any suitable or preferred size or form, but preferably has its lower ortion in the form of an inverted cone, as shown, and having an upper portion 20 of cylindrical form, the walls of the vessel having such slopes that the velocity of the upwardly flowing water therein will decrease. at such rate that the particles of abrasive material will assume positions at different levels therein, according to their size, as is now well understood. The upper portion of the vessel is made cylindrical, as shown, in order to enable the use of a smaller tank and also to secure a denser mixture of the finer grades with a smaller total quantity of fine material in the grading vessel. The progressive decrease of upward velocity of the water in said cylindrical portion in order to facilitate such grading and insure a more uniform density and distribution of the various grades is secured by withdrawing a portion of the mixture through a secondary outlet pipe 21 which discharges into the gutter 3 at 22 and whose inner end is connected inside of the cylindrical portion 20 of the grading vessel, to a vertical pipe 23 provided with a plurality of small ports or openings 24 at different levels, each of which admits a small portion of the mixture at that level. The secondary outlet pipe 21 passes through the wall of the grading vessel near its top, and is provided with an open-ended branch 25 which extends above the top of the grading vessel to break the siphon and decrease the head which forces the mixture through said outlet pipe.

The discharge of a small quantity of liquid at each of the openings 24 results in progressively reducing the upward velocity in the cylindrical portion 20, by a series of steps so that the finer grades assume a greater density and a more even distribution than would otherwise be the case, and the height of this portion of the vessel can be greatly reduced while still giving a wide variation in the size of material held therein. There is therefore a gradual reduction in velocity upwardly, due to the plurality of outlet openings 24 and consequently at each level a certain grade or size of material gradually accumulates, the grades becoming gradually finer and finer upwardly between successive levels as the interstitial velocities of the particles decrease. The several zones between the successive outlet openings 24 therefore form a series of traps, each accumulating its own particular grade of fine material-and storing it for later use, each grade becoming denser and denser as the grading operation continues. The cylin- "drical portion 20 of the grading vessel therefore forms a supplemental grader superposed upon the lower or conical portion of the vessel and having substantiall Y the same effect as though it were conical, but because of its cylindrical form the total volume of liquid to be taken care of is considerably reduced so that the vessel itself can be smaller and its supporting structure lighter than would otherwise be the case, thereby decreasing the cost and reducing the space re uired.

he mixture withdrawn throu h the outlet pipe 21 flows to the sump an returns to the circulation. and the material therefore is not wasted. It also assists in washing out and keeping clean the gutter 3, thereby reducing the amount of clean water otherwise required forthis purpose, and, indeed, because of its specific gravity it is more eflicient for washing out the coarse sand in the gutter 3 than if thewater wereclear.

Provision is made for separately withdrawing the several grades from the grading vessel by providing a plurality of withdrawal pipes, the number thereof depending upon the number of grades which are to be separately withdrawn. Three such pipes are shown in the drawings and are marked respectively 26, 27 and 28, and all pass out through the walls of the grading vessel at the same level, thereby contribut ing to the maintenance of a constant head or pressure. As shown, these pipes are all connected to a common casting or member 29 which is secured to the wall of the grading vessel and is provided with a plurality of inwardly projecting tubular portions, one

, for the attachment 'of each of the withdrawal pipes. pipe 26 has its inner end opening near the bottom of the grading vessel 11, in the coarse sand zone; the withdrawal pipe 27 has its inner end opening at a point between the top and bottom of the cone, in the intermediate sand grade zone, while the withdrawal pipe 28 is in the form of a flexible hose or pipe of sufficient length to reach to the bottom and also to the top of the grading vessel and. having its end verticall movable, as hereinafter described, so that it can take the sand out of said grading vessel provided with an outlet conduit 31 leading to the sump 6. Pivotally mounted adjacent to the discharge openings through the casting 29 are a plurality of discharge pipes, marked respectively 26*, 27 and 28, which new mounted and have their inner ends so, formed that when lowered each fits over its respective discharge opening in the casting 29 and forms a conduit for conducting the discharge through that particular. opening. to a trough 32 extending through the outer wall of'the vessel or casing 30 and leading to the spout 33 which projects over the grinding table 1, preferably between the runners 2, and which is perforated; to discharge the mixture of sand and water onto the table. The discharge pipes 26", 27 and 28 are preferably bent to substantially goose-neck form, as shown, althoughthisis As shown, the withdrawal not essential, and are arranged to be swung upwardly. about their pivots by suitable operating cords 26 27 and 28', each of which is knotted so that it ma he slipped into a notch in a holding mem r 34, or provided with other meanswhereby the pipe can be held in elevated position.

When all of the pipes are elevated the mixture discharging through each of the withdrawal pipes 26, 27 and 28 returns through conduit 31 to the sump 6, but whenever one of said pipes is lowered, the mixture coming through the corresponding discharge pipe'is conducted to the table. By this arrangement, therefore, the desired grade can be conducted to the. table by merely lowering the proper discharge pipe 26, 27 or 28, and that particular grade will continue to be supplied to the table as long as that discharge pipe remains in its lowered position. v

This arrangement does away entirely with valves, which; are subject to severe wear by the sand mixed with the water, while at the same time placing the supply of the different grades of sand to the table entirely within the'control of the operator.

In order to bringj the inner open end of withdrawal pipe 2 8jti' idiiferent heights for withdrawing different "grades, as is necessary during the finishing operation, its free said end. This weight is connected by a cord or cable 40, which passes over a sheave or pulley 41, to afloat 42 in a .cylinder 43 supported at the side of the grading vessel 11. Connected-to the lower end of cylinder 43 is a water pipe 44 which preferably extends. to a point adjacent to the front of the table where it is'provided with a suitable three-way cook or valve 45" which has a discharge outlet and also communicates with a water supply, so that by merely setting said cock to the proper position water ma be either admitted to the cylinder 43 to li the float 42 and allow theweight 39 to sink the end of the flexible pipe 28, or to discharge water from cylinder 43 and allow the float 42 to drop and elevate the end of flexible pipe 28 in the grading vessel.

Preferably, a second cable or cord 46 is connected to the float 42 and passes over pulleys 41 and 47.to the opposite side of the tank where it operates a suitable indicating means, shown as a-weight38 which is'preferably held at the same level as the weight 39, so that the position of weight 38 indi cates the exact positionof the inner open i end of pipe 28in the grading vessel. Preferably, also, suitable means is provided-for indicating to the operator the density of the mixture within the grading vessel. The indicating means'shown comprises a suitable displacement member or hydrometer 54 which is so constructed and so loaded end is supplied witha weight 39 for sinking at different levels and preferably covered with different colored glasses, such as red for the upper, white for the intermediate and green for the bottom window.

The position of the hydrometer 54 depends upon the density of the mixture in which it is immersed. It is substantially entirely immersed in the liquid and rises as the amount of abrasive material in suspension increases, and falls as the amount of abrasive material decreases. It is of such a size vertically of the grader that it is affected by changes in density in practically only a single grade or zone of material, which may be, say, the coarse sand, in which case the indicator 60 gives a visual indication of the condition of the coarse sand mixture, that is, its density and, roughly, the amount of material therein, while. variations in the density of the other grades do not affect the hydrometer. This indicator also advises the grinding foreman of the condition of each table in a series of tables, that is, whether a table is just beginning the rough or facing operation, whether it is at the intermediate 35 rindingoperation, or whether it is doing fine grinding or finishing. By watching the changes in the indicators on each of a series of grindingtables the foreman can tell at a glance whether the tables are properly supplied with sand as well as the character of the operation then in progress, and can so adjust the amount of coarse sand used at each facing operation that the proper mixture will be insured.

Preferably, also, it is desirable to control or regulate the speed of rotation of the pump, and therefore of the driving means for the same, in such manner as to cause it to deliver a uniform supply of abrasive material mixed with the liquid into the grading vessel, irrespective of variations in density of the mixture therein, or variations in any other conditions which affect the velocity of flow, as ully illustrated and described in v my application Serial No. 809,695, herein-. before referred to, but as this forms no part of the invention herein claimed, it need not be illustrated and described. r

In the use of the apparatus described, the sump 6 and grading vessel 11 are first filled with liquid, such as water, and the motor set into operation to start the pump circulating the water. All of the discharge pipes 26*, 27 and 28' are elevated so that the liquid is discharged therefrom into the casing or veswhile for the sel 30 and returns through pipe 31 to the sump 6. Liquid is also discharged from the secondary outlet 21. A measured charge of sand sufiicient for a complete single grind ing operation from roughing to finishing is measured into the hopper 17 and delivered or washed into the sump at such a rate as to secure the proper density of the mixture and so that all of the sand gets-into the circulation system before the fine grinding begins. This sand is carried up into the grading vessel and is graded according to its size, the larger particles assuming positions near the bottom of the cone and the grades becoming finer and finer up through the conical and cylindrical portions of the vessel. All dirt, scum and material too fine for use is washed out through the overflow 12, and a portion of the mixture in the cylindrical part 20 is also delivered through the outlet 21 to the gutter 3 and returns to the sump. Since the coarse sand at first predominates, the mixture in the bottom of the cone 11 is dense so that the hydrometer 54 rises and moves the indicating lamp 60 to a position opposite the lower window casing 61 and displays a green light, which means that the system is supplied with the proper quantity of coarse sand for economically effecting the rough grinding.

By properly manipulating the several discharge pipes 26, 27 and 28, the several grades may be supplied from the grading vessel to the grinding table separately and independently and in any order and to any extent or degree desired. During the rough grinding, pipe 26 is lowered to supply coarse material from near the bottom of the gradin vessel into the trough 52 from which it flows to the spout 53 and is discharged upon the table, while the discharge pipes 27 and 28 are in elevated position so that the mixture discharging through withdrawal pipes 27 and 28 returns to the sump 6. When the whole batch of sand has been introduced into the sump and carriedto the grader the hydrometer is in its highest osithereafter a considerably finer grade of material is delivered to the table from a lever between the to and bottom ofthe cone, nal finishing operation the pipe 27 a is lifted and the pipe 28" is lowered, thereby discharging intgthetrough 52 material coining through the withdrawal piga 28.

uring the rough grinding some of the sand is thrown off-thetable in the condition in'which it is received, especially (lur- 'ing the stage of adjusting the runners into full contact and'full pressure with the table. The material so thrown off the table into the gutter 3 returns to the sump and back again into the-grading vessel. During all of the grinding operations some of the sand is broken up by the grinding action, produc- 1ng grades of smaller size, some mud and some material too fine for further use.

During each cycle of the circulation the through the overflow pipe 12 is replenished by a Water supply pipe 6, which discharges into'the sump 6 and is preferably provided with a ball float controlled valve 6" arranged to turn on the water supply whenever the level of liquid in the sump falls to a certain level, say to a level one-third the depth of the sump below. the overflow therefrom. This prevents the pump from wholly depletthe liquid and abrasive supply in the sump. The final finishing is done with the several rades ofabrasive material held in sus- P6I1S10I11I1 the cylindrical portion 20 of the grading vessel. The inner open end of pipe 28 is placed in the zone of the grade with which: the finishing operation is to begin,

say, at the bottom on the cylindrical portion 20 of the vessel 11, and may then be raised at Intervals by a series of steps or jumps so as to deliver to the grindingtable successively portions of the material of liner and finer grades, until the smoothing operation is completed. Preferably, however, the open end of pipe 28 is moved upward at a uniform rate of speed, traversing in turn each of the various levels in the cylinder 20 finest grade at the top of the cylinder.

in a series of imperceptible changes, so that thegrade of material supplied to the table changes gradually from the intermediate grade near the top'of the cone to the very To secure uniform results and also to regulate the time of grinding, the three-way cock 45 is so set as to discharge the water from cylinder'48 at just suchia. rate that the float 42 Wlll raise the open end of pipe 28 at the proper rate of speed. If desired, the outlet of cook 45 may be made of such a size as to constitute thelimit of the maximum rate of discharge. This does away with the necess ty of watching the apparatus closely durmg the fine grinding and adjusting the flexible ipe at'proper intervals for changes in gra' es.-- I

,A's' heretofore stated, the flexible withdrawal. pipe '28 is preferably made long.

1 enoughzto extend to both the bottom and amp swa v s that f.

desired it may be used to deliver to the grinding table abrasive material from any zone or level in the grading vessel. Consequently, said pipe can be utilized at any time to deliver an extra supply of material in addition to that delivered by either of the pipes 26 or 27. i It may also be used as the sole source of supply for the entire grinding operation from the beginning of the roughing operation to the final finishing. Cylinder 43 is long enough to allow a sufficient movement of float 42 to move the end of pipe 28 from the bottom to the top of the grading vessel, and bystarting the operation with the pipe 28 at the bottom andduring one operation than duringthe other.

The time required for facing a rough surface, that is, to produce a perfect plane, varies according to the roughness and straightness of the rough glass, while the time required to bring the rough face of the plane surface so produced to afine and uniform smoothness ispractically constant. I prefer, therefore, to supply the abrasive for the grinding operation from a fixed level, as from the pipe 26, so as to insure that only the coarsest material is used for this operation, but the finishing is best accomplished by supplying the various grades from the beginning of the finishing operation to the completion thereof, successively in a definite progression and length of time.

During the entire grinding operation, from roughing to finishing, the abrasive material is in suspension in a liquid and is circulated repeatedly from the grading vessel to a grinding table, then to the sump, and back to the grading vessel; slimes and fine material being separated during the course of each cycle. After the introduction of the initial supply or charge of abradtion, and supplies the necessary amount of fine sand for the finishing. vessel supplies only a single table, and therefore the conditionsv of water supply, sand supply, and pressure or head areat. all

The grading times uniform. ,By having all of the discharge openings from the grading vessel open at all times, by maintaining a uniform waste overflow from the upper portion of v the vessel, and by taking all of the abrasive mixtures to the grinding table out of the grading vessel at the same level, uniformity in pressure and head is insured.

As the grinding operation proceeds the density of the coarse mixture in the grading vessel decreases and the hydrometer 55 sinks, thereby raising the lamp 60 and changing the-indication from green to white and then-to red, thereby indicating to the foreman the particular stage of grinding operation. I

When the grinding operation is completed all of the discharge pipes 26", 27 and 28 are raised and the mixture is allowed to flow from the grading tank into the vessel or casing 30 from which it flows to the sump 6, thereby enabling the circulation to be continued between successive grinding operations. or during the time that the glass on the table is being changed.

Usually, upon starting a given system a slight excess of coarse sand is supplied with each of the first few batches, so as to insure suflicient'coarse sand for the rough grinding, and which excess is broken up on the table to form finer grades which accumulate in the cylindrical portion 20 of the grading vessel and form a reserve supply of fine material which can be drawn upon in case it is necessary or desired to finish or re-finish a side of glass without first going through the grinding operation. After a sufficient excess of fine sand has accumulated in the vessel 11 the measured batches of sand thereafter introduced are calculated so as to be just suflicient for one complete grinding operation. The mud and fine material washed out through the overflow during each grinding operation are equal in quantity to the new batch of sand supplied to the system for each grinding operation. By operating the system in this manner each batch of sand introduced into the system is graded and supplied to the table under con ditions identical with those in the preceding operation, and these conditions are maintained during the entire course of each and every grinding operation.

\Vhat I claim is:

1. Apparatus for grinding and smoothing plate glass, comprising in combination, a single grinding table, and a sand grading apparatus and circulating system associated therewith and supplying said individual grinding table alone, and comprising grading apparatus receiving the sand and water from the grinding table and arranged to separate the fine from the coarse sand, conduits arranged to repeatedly circulate the coarse sand so separated out to the grinding table and back to the grading apparatus for re-grading, whereby the sand is progressively reduced in size, progressively finer grading is etfected, and the necessary fine sand for finishing is produced, means for separating the fine sands into various grades and storing the same, and means for conducting said fine grades or fractional parts thereof successively to the grinding table, independently of each other.

2. Apparatus for grinding and smoothing plate glass, comprising in combination, an individual grinding table, and a sand grading apparatus and circulating system associated therewith and supplying said individual grinding table alone, and comprising grading apparatus for receiving fresh sand and the overflow from the grinding table and arranged to preliminarily grade the fresh sand and separate the fine from the coarse used sand, conduits arranged to repeatedly circulate the coarse sand so separated out to the grinding table and back to the grading apparatus for re-grading, whereby the sand is progressively reduced in size, progressively finer grinding is effected,

and the necessary sand for finishing is formed. means for separating said finer sands into various grades and storing the same, and means for conducting said finer grades or fractional parts thereof successively to the grinding table. independently of each other.

In testimony whereof, I have hereunto set my hand.

HALBERT K. HITCHCOCK.

\Vitness C. S. LAMB. 

